The present invention relates to a solid polymer fuel cell system and a method for humidifying and adjusting the temperature of a reactant gas stream therefor. More particularly, the invention relates to a fuel cell system and method in which heat generated by the fuel cell and water vapor in a reactant stream exhausted from a fuel cell are used to heat and humidify a reactant gas stream prior to introduction into the fuel cell. The invention is particularly suitable for air-cooled fuel cell systems or systems which employ near ambient pressure air as the oxidant gas supply.
In solid polymer fuel cells which employ an ion exchange membrane (typically a proton exchange membrane) as the electrolyte, the water content of the membrane affects the performance of the fuel cell. The ion conductivity of the membrane generally increases as the water content or hydration of the membrane increases, therefore it is desirable to maintain a sufficiently high level of hydration in the membrane during fuel cell operation. For this reason, the reactant streams are typically humidified prior to introduction into electrochemically active regions of the fuel cell.
The capacity of reactant gases to absorb water vapor varies significantly with changes in temperature and pressure. Therefore, it is preferred to humidify the reactant gas streams at or as near as possible to the operating temperature and pressure within the fuel cell. If the reactant gas is humidified at a temperature higher than the fuel cell operating temperature this can result in condensation of liquid water occurring when the humidified reactant gas enters the fuel cell. Condensation may cause flooding in the electrodes which may detrimentally affect fuel cell performance. Conversely, if the reactant gas stream is humidified at a temperature lower than the fuel cell operating temperature, the reduced water vapor content in the reactant gas stream could result in membrane dehydration and damage to the membrane. Thus, the reactant streams are often heated and humidified prior to introduction into the fuel cell.
Various approaches have been used to increase the humidity of reactant gas streams supplied to fuel cells. For example, some conventional solid polymer fuel cell systems humidify a reactant gas stream by flowing the reactant gas stream and liquid water on the opposite sides of a water-permeable membrane. Water from the liquid stream is transferred through the membrane, thereby humidifying the reactant gas stream. The pressure, temperature, flow rates and path length through the liquid water-to-gas humidifier can be adjusted to give the desired water vapor content in the reactant stream. The water is preferably de-ionized to prevent ionic contamination of the fuel cell membrane electrolyte.
For example, U.S. Pat. No. 5,547,776 discloses a fuel cell system which includes a membrane humidifier for a reactant gas stream with the humidifier preferably using de-ionized liquid water.
Such liquid water-to-gas humidifiers are commonly used in solid polymer fuel cell systems in which water is used as a cooling fluid, as the cooling water is a convenient source of water for the humidifier. Japanese Patent Publication No. 09-055218, for instance, teaches membrane humidification of the reactant gas supply stream using the warm water of the liquid coolant. The humidifier employed may be a separate module in the solid polymer fuel cell system. Alternatively, it may be incorporated as an assembly in the fuel cell stack itself, such as between the end plates of fuel cell stack. U.S. Pat. No. 5,382,478 discloses such an assembly in the fuel cell stack wherein a liquid water-gas humidification section is located upstream of the electrochemically active section of the fuel cell. U.S. Pat. No. 4,973,530 shows another embodiment comprising a liquid water-gas humidification section. In fact, the humidification section can be even more closely associated with the electrochemically active section of the fuel cell stack. For instance, PCT International Publication No. WO 96/24958 shows that humidification sections can be created by leaving portions of the electrode surfaces uncoated with catalyst. Thus, humidification of the anode stream by the cathode stream can occur across the membrane electrolyte material in these areas. However, these uncoated portions are not electrochemically active and thus represent inefficient use of electrode and polymer electrolyte membrane surface area.
The reactant streams exiting the fuel cell or fuel cell stack typically contain product water, as well as water vapor which was present in the humidified stream delivered to the fuel cell. In particular, the oxidant stream, as it travels through a fuel cell, absorbs water that is produced as the product of the electrochemical reaction at the cathode.
In some fuel cell systems, the product water from the fuel cell is condensed from the exhaust reactant streams and is collected and then used for reactant stream humidification and/or cooling of the fuel cell. In such systems, the water in the exhaust streams is typically collected in the liquid phase and is generally combined with a larger liquid cooling water supply as shown, for instance, in U.S. Pat. No. 5,200,278.
Other conventional approaches for humidification of reactant gas streams prior to introduction into fuel cells include the injection of water vapor or atomized water droplets into the reactant streams (as shown, for instance, in U.S. Pat. No. 5,432,020), and exposing a reactant gas stream directly to water in an evaporation chamber to permit the stream to absorb evaporated water. Japanese Patent Publication No. 07-176313 shows a fuel cell system where pure water is used to humidify the reactant gas supply streams and where the oxidant stream exhaust is used to heat the incoming oxidant reactant gas stream in a heat exchanger.
Solid polymer fuel cell systems are typically liquid-cooled rather than air-cooled if higher power densities (power output capability per unit volume) are required. The reason is that their cooling systems must shed a significant amount of heat at relatively low temperature (circa 80xc2x0 C.) with respect to ambient temperature. In addition, the use of liquid- as opposed to air-cooling allows the fuel cell stack cooling channels to be made smaller and hence a lower overall stack volume can be obtained. However, air-cooled fuel cell systems may be preferred in many applications where power density is less important. However, for humidification purposes, such air-cooled systems cannot rely on the coolant as a supply of water.
It is well known that transport across a semi-permeable membrane is substantially more efficient from the liquid phase than from the gas phase. Nonetheless, it is also well known that a significant, but lesser, transfer of species can take place across a suitable semi-permeable membrane, from the gas phase, even against an absolute pressure differential, as long as there is a significant partial pressure difference of the species across the membrane. For instance, U.S. Pat. No. 3,494,174 discloses a semi-permeable membrane exchange apparatus for transferring species from one gas stream to another, against an absolute pressure difference, for use in gas chromatographs. However, predominantly gas-gas phase membrane exchange humidifiers have generally not been contemplated for use in humidifying the reactant gas supply streams of fuel cells. Presumably, the heating and humidity requirements for fuel cell reactant gas supply streams were not viewed as achievable in gas-gas humidifiers.
In fuel cell systems which employ conventional reactant gas supply humidifiers, the desired heating of the supply stream may be accomplished simultaneously with the humidifying. For instance, heat is exchanged across the membrane in any liquid water-gas membrane humidifier supplied with warm liquid water from the circulating coolant.
Sometimes it is not necessary to use a humidifier to introduce water vapor into both the oxidant and fuel reactant streams. In systems in which the reactant gas is partially or fully recirculated through the fuel cell stack, it may not be necessary to humidify the additional fresh gas stream which is also supplied, if the product water carried back to the inlet by the recycled stream is sufficient to maintain adequate hydration of the ion exchange membrane. U.S. Pat. No. 5,543,238 illustrates partial recirculation of the fuel cell exhaust gas to heat and humidify the incoming reactant stream. Additional apparatus for performing the recirculation is necessary however. In direct methanol fuel cells, the methanol fuel stream may be supplied as a dilute aqueous solution in which case further humidification of the fuel stream is not required. Similarly hydrogen-containing reformate streams (obtained, for example, by reforming a fuel such as methanol, natural gas or butane), which may be supplied as the fuel stream to a fuel cell, typically contain sufficient residual water vapor from the reforming process.
While many varied fuel cell systems appear in the art, typically at least the oxidant gas supply stream is humidified and heated prior to introduction into a fuel cell. The required humidification and heating apparatus typically adds to the complexity of the fuel cell system, as it generally includes additional system components, such as a humidification water pump, piping, water reservoir and filtration unit, in addition to the humidification module and heater. It can also add to the parasitic load of the system as power is required for operating pumps and heating the stream.
Accordingly, a simpler and more energy efficient means for pre-heating and humidifying reactant supply streams in a solid polymer fuel cell systems is desirable.
In the present method for humidifying and adjusting the temperature of a reactant gas stream supplied to a solid polymer fuel cell, an exhaust reactant stream from a fuel cell is used to heat and humidify a reactant stream supplied to the fuel cell. The method is particularly suitable for use in conjunction with a fuel cell in which the electrochemical reaction is exothermic and produces water. In this case, exhaust reactant streams exiting the fuel cell will typically be warmer and have a higher partial pressure of water vapor than the supply reactant streams.
The reactant gas supply streams for a solid polymer fuel cell include a fuel supply stream and an oxidant supply stream that are supplied to fuel and oxidant inlet ports of the fuel cell respectively. The solid polymer fuel cell also typically has both a fuel exhaust stream and an oxidant exhaust stream exiting the fuel cell via fuel and oxidant exhaust ports, however one of the reactants may be essentially dead-ended with optional intermittent venting of inert components. A method for heating and humidifying a reactant gas supply stream for the fuel cell then consists essentially of:
a) providing a combined heat and humidity exchanger comprising a supply stream chamber, an exhaust stream chamber, and a water permeable membrane separating the two chambers;
b) directing the reactant gas supply stream through the supply stream chamber upstream of the fuel cell reactant gas inlet port; and
c) directing the reactant gas exhaust stream from the reactant gas exhaust port through the exhaust stream chamber
whereby water and heat are transferred from the reactant gas exhaust stream to the reactant gas supply stream across the water permeable membrane. The reactant supply stream is thus heated and humidified before entering the fuel cell.
Preferably, the oxidant supply stream is the reactant gas supply stream which is heated and humidified. (Indeed, in certain fuel cell systems, the fuel supply stream is not humidified at all.) The method is particularly suitable for fuel cell systems which employ near ambient pressure air (i.e. below about 300 mbar pressure) as the oxidant gas supply. As no liquid water supply is needed for humidifying the reactant gas supply stream, the method is also particularly suitable for fuel cell systems which are air-cooled.
Further, the exhaust stream preferably used to heat and humidify the reactant gas supply stream is the oxidant exhaust stream. Thus, a preferred embodiment involves heating and humidifying the oxidant supply stream using the oxidant exhaust stream. In such an embodiment, it may be desirable to set the flow rate of the oxidant supply stream in the range of from about 6 to 90 L/minute in order to obtain efficient transfer of water vapor and heat across the water permeable membrane.
Generally, in order to obtain efficient transfer of water vapor and heat from an exhaust stream to the supply stream across the water permeable membrane, it is desirable to direct these streams on opposite sides of the membrane in a counterflow configuration. Further, it is desirable to employ other design features in the combined heat and humidity exchanger construction and to set the relevant system operating parameters, including the flow rates of the selected reactant gas supply stream and exhaust stream, such that the transfer of water vapor and heat is generally more efficient.
A dimensionless parameter, herein denoted as R, may be useful in selecting design features and operating parameters for efficient water vapor and heat transfer. R is defined as the residence time to diffusion time ratio for a water molecule in a chamber in a combined heat and humidity exchanger or CHHE. Herein, the diffusion time is defined as the time it takes for a water molecule to diffuse over the depth of a given chamber to the membrane in the CHHE. The residence time is the mean time that a water molecule spends in a given chamber. Empirically, it has been found that R is preferably in the range from about 0.75 to 3 for water molecules in either the supply stream or exhaust stream chambers. Preferably R is in this range for water molecules in both chambers.
A solid polymer fuel cell system includes a solid polymer fuel cell and apparatus for heating and humidifying a reactant gas supply stream. The reactant gas supply stream can be either a fuel supply stream or an oxidant supply stream of the fuel cell. The fuel and oxidant supplies are fluidly connected to fuel and oxidant inlet ports of the fuel cell respectively. The fuel cell system also has at least one reactant gas exhaust port. An apparatus for heating and humidifying the reactant gas stream is a combined heat and humidity exchanger, CHHE, consisting essentially of:
a) a supply stream chamber having an inlet and outlet wherein the reactant gas supply is fluidly connected to the supply stream chamber inlet, and the supply stream chamber outlet is fluidly connected to a reactant gas inlet port of the fuel cell;
b) an exhaust stream chamber having an inlet and outlet wherein a reactant gas exhaust port of the fuel cell is fluidly connected to the exhaust stream chamber inlet; and
c) a water permeable membrane separating the supply stream chamber and the exhaust stream chamber whereby water and heat can be transferred from a reactant gas exhaust stream to the reactant gas supply stream across the water permeable membrane.
As above, the reactant gas supply stream in a preferred solid polymer fuel cell system is the oxidant supply stream and the oxidant exhaust stream is the preferred reactant gas exhaust stream. Thus, the system is configured so that in operation the oxidant supply stream is heated and humidified by its own exhaust.
The basic construction of a CHHE can be conventional. The water permeable membrane therein has oppositely facing major surfaces, and the reactant gas to be supplied to the solid polymer fuel cell is directed in contact with one surface of the membrane while simultaneously the desired exhaust stream from the fuel cell is directed in contact with the opposite surface of the membrane. Preferably, the streams are directed on opposite sides of the membrane in a counterflow configuration. Prior to contacting the membrane, the reactant gas supply stream has a lower temperature and a lower partial pressure of water vapor than the exhaust reactant gas stream. Water and heat are transferred through the membrane from the exhaust reactant gas stream to the supply reactant gas stream.
Preferably the membrane is impermeable to the reactant, and more preferably is substantially gas impermeable. This prevents reactant portions of the supply and exhaust streams from intermixing. Suitable membrane materials include cellophane and perfluorosulfonic acid membranes such as Nafion(copyright).
The CHHE is preferably external to the solid polymer fuel cell and may comprise multiple supply chambers and/or exhaust chambers. The supply reactant stream may be directed through the supply chambers in parallel or in series, and the exhaust reactant stream may directed through the exhaust chambers in parallel or in series.
A CHHE may be connected to a fuel cell stack, or may be connected to a single fuel cell or a plurality of fuel cells. Preferably the CHHE is of a plate-and-frame type or a multiple plate-and-frame design, although other structures, such as, for example, a jelly-roll configuration or tube bundle configuration may be used.
In a preferred embodiment of the invention, a separately housed modular CHHE is used that can also optionally be installed in direct thermal contact with a fuel cell stack. In this way, the CHHE also benefits from the heat produced by the fuel cells in the stack. Preferably the CHHE contacts and can receive heat from each fuel cell in the stack. If a CHHE is installed as a module, detachably connected to the stack, it can be easily removed and replaced if servicing is required, without disassembling the fuel cell stack. In another generally less preferable embodiment, a CHHE may be incorporated between the end plates (but not between the bus plates) of the fuel cell stack.
As mentioned above, certain design features of a CHHE are advantageous insofar as water vapor transfer and also heat transfer efficiency are concerned. For instance, a CHHE can comprise flow channels for the supply and exhaust streams. It can be advantageous for the depth of the channels in either the supply stream chamber or the exhaust stream chamber to be in the range from 0.05 to 0.25 centimeters (0.02 to 0.1 inches). A perfluorosulfonic acid membrane about 0.018 cm (about 0.007 inches) thick is a suitable water permeable membrane for a CHHE.
In the embodiments described above, the reactant gas supply and exhaust stream may be an oxidant stream, such as for example an oxygen-containing gas stream, most preferably air, or may be a fuel stream, such as for example a hydrogen-containing gas stream. In preferred embodiments the heat-and humidity exchanger is used for gaseous oxidant streams, and a CHHE is used to humidify and adjust the temperature of an air stream supplied to a solid polymer fuel cell.
Thus, it is possible to exclusively use an exhaust stream from a solid polymer fuel cell to heat and humidify a reactant stream supplied to the fuel cell via a membrane exchange apparatus, without the need to condense and collect water from the stream for subsequent use for humidification. As described above, an exhaust stream, which has been heated during its passage through the fuel cell may be used to heat the supply reactant gas stream. Optionally, heat generated by the fuel cell is also used to directly heat the reactant supply stream before it enters the fuel cell. Thus, the heat and water vapor generated by the exothermic reaction in the fuel cell is used to directly heat and humidify a reactant supply stream, which can reduce or eliminate the need for conventional pre-heating and humidifying systems.